1. Field of the Invention
This invention relates to a semiconductor laser device to be used in the field of optical telecommunication, optical amplification or optical instrumentation as a source of light having a wavelength in a band between 0.9 and 1.1 .mu.m.
2. Prior Art
There have been proposed semiconductor laser devices comprising an InGaAs strained quantum well layer grown on a GaAs semiconductor substrate as an active layer and designed for use as a source of light having a wavelength in a band between 0.9 and 1.1 .mu.m.
Some of the promising applications of such semiconductor laser devices include pumping light sources for rare earth ion doped optical fiber amplifiers purposes and short wave light sources for light having a wavelength between 0.45 and 0.55 .mu.m, where such a semiconductor laser device is normally used in combination with one or more than one nonlinear optical devices.
In the field of pumping light source, semiconductor laser devices of the type under consideration are expected to be used in the near future as feasible sources of light (with a wavelength of 0.98 .mu.m) for optical fibers doped with erbium designed for amplification of light (having a wavelength near 1.5 .mu.m).
When a semiconductor laser device of the above described type is used as an pumping light source, it is optically connected to the small diameter optical waveguide of such as an optical fiber so that light emitted from the semiconductor laser device with a high output level of tens of several milliwatts should be effectively coupled into the optical waveguide.
The semiconductor laser device is required to oscillate to emit a beam in fundamental transverse modes to show a stable single lobe profile.
Various laser structures have been proposed for semiconductor laser devices to oscillate in fundamental transverse modes and, of these, a so-called ridge type laser structure is popularly used for InGaAs strained quantum well type semiconductor laser devices.
The reason why a ridge type laser structure is popularly used is that it can be suitably used for the above identified applications without requiring a strained active layer to be exposed or an exposed active layer to be buried in some other semiconductor layers.
Recently, a number of papers have reported the use of an InGaAs strained quantum well type semiconductor laser device comprising an InGaP clad layer in place of an AlGaAs clad layer.
Major advantages of using an InGaP clad layer in a semiconductor laser device are described in detail in Papers 1 through 4 listed below and a semiconductor laser device comprising such a clad layer is disclosed in Japanese Patent Laid Open No. Hei 3-222488.
Paper 1: J. P. Wittke and Ladany, J. Appl. phys., 48(1977) 3122. PA1 Paper 2: J. Buns, IEEE J-QF QF-19(1983) 953. PA1 Paper 3: T. Ohtoshi et al., Solid-State Electron., 30(1987) 627. PA1 Paper 4: R. Lang, IEEE J-QF QF-15(1979) 718-726.
As described in the above papers, an InGaAs strained quantum well type semiconductor laser device having an InGaP clad layer allows wet etching to be carried out at a very high selection ratio.
Therefore, a high precision ridge type laser structure can be realized for a laser device of the type under consideration by inserting a very thin GaAs layer to part of an InGaP clad layer and then selectively etching a portion of the InGaP clad layer lying on the GaAs layer.
FIG. 1 of the accompanying drawings is a schematic sectional side view of a semiconductor laser device prepared in an above described manner.
The semiconductor laser device comprises an n-InGaP lower clad layer 12, an InGaAs strained quantum well active layer 13, a GaAs etching stop layer 14, a p-InGaP upper clad layer 15, a rib-shaped p-InGaP clad layer 16 for providing a ridge, a p-GaAs contact layer 17 and a pair of polyimide layers 18a and 18b sequentially formed on an n-GaAs semiconductor substrate 11. The device also comprises a metal p-electrode 19 covering the upper surfaces of the contact layer 17 and the polyimide layers 18a and 18b and a metal n-electrode 20 covering the lower surface of the semiconductor substrate 11.